The present invention relates to a method of producing such a formed product with good heat resistance and shape memory characteristics.
Ethylene-propylene copolymers, for example ethylene-propylene-diene copolymers, etc. are elastomers which are widely used as rubber materials. However, since these elastomers have relatively poor heat resistance, they are usually cross-linked after having been formed into desired shapes.
They are sometimes blended with cross-linkable polymer components such as an ethylene-vinyl acetate copolymer and then cross-linked by heating. However, since such cross-linkable polymers are usually mixed with the elastomers at high temperatures of about 200.degree. C. or more, this may cause the cross-linking reaction to take place before the composition has been formed into desired shapes. This in turn makes it difficult to mold or form the resulting blends into desired shapes. In other words, such elastomer blends which have been subjected to premature cross-linking have poor moldability and formability. Accordingly, a polymer composition having, not only high heat resistance, but also good moldability and formability cannot be obtained.
For instance, Japanese Patent Publication No. 59-50172 discloses a resin composition comprising (a) a copolymer, such as an ethylene-vinyl acetate copolymer, (b) an ethylene-.alpha.-olefin random copolymer and (c) a polymer such as crystalline polypropylene, in proper proportions, which is cross-linked to have a boiling xylene-insoluble gel percentage and a melt index kept at certain levels. The cross-linking is conducted by irradiation of electron beam. However, since this composition is blended at as high a temperature as 240 C, the component (a), such as an ethylene-vinyl acetate copolymer cannot be added in a large amount. Therefore, the resulting composition has only 0.1-60 weight % of a boiling xylene-insoluble gel percentage after cross-linking. Because of this low gel percentage, this composition becomes sticky when heated to a high temperature.
Further, Japanese Patent Publication No. 62-22772 discloses a high-orientation film made of (a) 93-30 weight % of at least one polyolefin selected from the group consisting of an ethylene-vinyl acetate copolymer, low-density polyethylene, high-density polyethylene, crystalline polypropylene and polybutene-1, and (b) 7-70 weight % of a thermoplastic elastomer made of an ethylene-.alpha.-olefin copolymer, which has a boiling xylene-insoluble gel percentage of 60 weight % or less, a melt index of 1.0 or less, a tensile rupture strength of 5 kg/mm.sup.2 or more and a temperature at a 20 % shrinkage ratio of 85.degree. C. or less. This film is produced by extruding a resin having the above composition through an annular die at 180.degree.-280.degree. C., solidifying rapidly, cross-linking by irradiation of high-energy beam, and then stretching. However, since the resin is blended and extruded at high temperatures, premature cross-linking inevitably takes places during the steps of blending and extrusion. In addition, to enable the stretching of the film, the level of cross-linking by irradiation with high-energy beam must be low, resulting in a low boiling xylene-insoluble gel percentage.
Besides heat resistance, it was found that some types of elastomer compositions have shape memory characteristics, and various proposals have been made directed thereto.
For instance, Japanese Patent Laid-Open No. 59-53528 discloses a shape memory norbornene polymer having a glass transition temperature of 10.degree. C. or more and a number average molecular weight of 1,000,000 or more. This polymer is molded and then deformed at a temperature lower than the molding temperature, and then this deformation of the molded product is set by cooling at a temperature lower than the lass transition temperature. When it is heated at a temperature between the glass transition temperature and the molding temperature, the deformed product to its original shape.
Japanese Patent Laid-Open No. 60-28433 discloses a synthetic copolymer, such as a butadiene-methyl methacrylate styrene copolymer, having a glass transition temperature higher than room temperature. Its cross-linked product has excellent shape memory characteristics and good formability.
Further, Japanese Patent Laid-Open No. 62-86025 discloses that a vulcanized rubber made of a polymer or its blend, such as a polychloroprene rubber having a glass transition temperature of -10.degree. C. or less, a melting point of 35.degree.-90.degree. C. and a crystallization degree of 10-50%, has good shape memory characteristics and good rubber elasticity at room temperature.
However, any of the above conventional shape memory polymer compositions are not satisfactory because of either one of the following problems:
(a) When a norbornene polymer is used, its molecular weight is very large, leading to poor moldability and formability.
(b) Because of high crystallization temperature, the original shape cannot be regained without applying high temperature.
(c) Special conditions are needed for molding and forming.